Upon tasting the beer for the first time, Matthew J. Farber was convinced that he had an infection in his fermentor. The assistant professor of biology and the director of the Brewing Science program at University of the Sciences in Philadelphia had made a pilot batch of beer on a SabCo system in the brewing lab using yeast harvested in the city. Assuming that, and based on previous experiences, the yeast collected—if it was up to the task of fermentation—would be wild, the class regularly uses the Sour American recipe from Michael Tonsmeire’s book, American Sour Beers. It’s a simple lambic-like beer with German Pilsner malt, wheat, oats, and a small amount of Willamette hops.
The students use this recipe because they want to focus on the yeast flavor, says Farber. The batch in question had a clear lactic-acid flavor, something that was not added in the brewing process and that had not shown up in any of the other pilot batches. Further confounding Farber was that the beer was the pale yellow he was expecting but completely clear.
“We spent a lot of time thinking there was a contamination and then spent so much time checking for microbial contamination. When we kept getting the same results, by the fourth batch, we realized that there wasn’t a contamination and that the yeast was making lactic acid in fermentation.”